Abstract of Funded Parent Project Rotator cuff disorders of the shoulder are particularly devastating in the aging population with tears present in 50% of people over 60. Specific development and maturation processes, along with cues from the mechanical loading environment, generate and remodel tendon structure and composition throughout life. These processes are regionally-dependent, suggesting a complex spatially-dependent regulation of tissue homeostasis. During aging, normal maturation processes and mechanical influences can result in accumulation of sub- rupture damage and ultimately to tendon degeneration. The focus of this application is to determine the mechanisms by which this might occur. The development of tendon structure is dependent upon collagen I assembly into fibrils and higher order assemblies. This process is controlled by interactions involving collagen V, a quantitatively minor yet critical regulatory component of tendon. Recent studies demonstrated that altered fibril/fiber structure due to decreased collagen V content results in an inferior dynamic response to load and inferior macroscale function. Furthermore, there was a differential regulation of structure by collagen V at the insertion site and midsubstance of the supraspinatus tendon. Other regulators of fibrillogenesis, such as small leucine-rich proteoglycans and quantitatively minor collagens (XI, XII and XIV), recently have been shown to be associated with age-related matrix remodeling. However, the regulatory role(s) of collagen V have not been elucidated. Therefore, the overall objective of this proposal is to elucidate the differential regulatory role(s) of collagen V at the insertion site and midsubstance of the supraspinatus tendon both at maturity and during the aging process. Our general hypothesis is that regulation involving collagen V changes with aging and results in site-specific regulatory roles due to altered content and distinct interactions resulting from differences in matrix molecules present and assembled at the two sites. We will test this hypothesis using targeted collagen V mouse models, which will define the role of an abnormal matrix in aging, as well as with novel collagen V inducible models, which will delineate the role of altered collagen V in the progression of tendon aging. This approach will provide an innovative template for tendon structure-function studies and insight about the critical regulatory roles of collagen V in tendon aging by (1) defining the site-specific alterations in structure, composition, dynamic processes and mechanical function during normal supraspinatus tendon aging, (2) elucidating the site-specific differential roles of collagen V in determining aging-associated declines in supraspinatus tendon properties, and (3) delineating site-specific hierarchical structure-function relationships as a function of collagen V content and aging utilizing sophisticated multiple regression models. This innovative approach will be coupled with sophisticated and innovative measures of mechanical (including fatigue) and organizational properties, together with compositional profiles, to derive a mechanistic understanding of the governing processes.